Thermoplastic polyurethanes are relatively easy to process into a wide variety of fabricated products. Unfortunately, however, the high temperature stability of these polyurethanes, as well as their stability in some commonly-used organic solvents, are less than might be desired. Accordingly, methodology has been developed to provide heat-induced crosslinking to convert thermoplastic polyurethanes into thermoset polyurethanes having the desired stability at high temperatures and in the presence of solvents. By way of illustration, U.S. Pat. No. 4,255,552 discloses thermoset polyurethane elastomers obtained by adding organic peroxides to a liquid polyurethane-forming composition prior to reacting the composition to form the polyurethane. The '552 patent teaches that the liquid polyurethane-forming composition containing "unactivated hydrogen peroxide" may be formed into a desired article and then heated to thermoset the article, or provided in solid form such as sheet, crumbs, or granules which are then formed into a desired article that is then thermoset by heating the article. The organic peroxides disclosed in the '552 patent are said to have a half-life of greater than one hour at 100.degree. C. Unfortunately, these peroxide-containing compositions are less stable than might be desired during melt processing or thermoforming of the polyurethane composition into the desired finished article, thus providing technology that is not commercially practical.
As an alternative to heat induced crosslinking of thermoplastic polyurethanes, their conversion into thermoset polyurethanes by irradiation is known in the art. A technical journal article entitled "Radiation Crosslinked Thermoplastic Polyurethane", published in the journal International Polymer Science and Technology, Vol. 19, No. 1, pp. T/6-T/9 (1992), discloses the production of such thermoset polyurethanes using a polyisocyanate and methacrylate monomer as a radiation-cross-linkable monomer. This technical journal article does not disclose the particular polyisocyanate used in making polyurethanes disclosed therein. Unfortunately, methacrylate is more heat sensitive than otherwise might be desired, causing a risk of premature cross-linking during storage or shipping and prior to the desired conversion of the thermoplastic polyurethane into a thermoset polyurethane. Further, not all polyisocyanates perform alike in irradiation-crosslinking of TPUs. Indeed, the present inventor has been unsuccessful in attempts to cross-link TPU formulations based upon aromatic polyisocyanates to provide a desirable article. Instead of cross-linking, the resulting article exhibits an undesirable discoloration.
In view of the above, there is a continuing need in the polyurethanes manufacturing community for a polyurethane-forming composition that is readily thermoset by cross-linking when desired, but is also less sensitive to unwanted heat-induced cross-linking during storage and prior to use than prior art polyurethane-forming compositions, most notably prior art peroxide, acrylate, and methacrylate-containing compositions. Such a composition desirably would provide advantageous processing capability, such as by extrusion, when the composition is in the thermoplastic state, and advantageous elevated temperature stability and solvent resistance when the composition is thermoset after formation into the desired product. The present invention provides one such desirable composition, together with processes for the production of the composition.